5.2 connectice tissue

Overview of Connective Tissue

The most diverse, abundant, and widely distributed tissue; functions: bind, support, and protect organs; acts as the glue that secures body structures together.
Examples: tendons (muscle to bone), ligaments (bone to bone), adipose tissue, cartilage, bone, and blood.
Common origin: all derived from embryonic mesenchyme.
Vascularity varies by type; regenerative capacity also varies: bone and blood are highly regenerative; cartilage is not.

Three basic components: cells, protein fibers, and ground substance; together form the extracellular matrix (ECM).
Diversity arises from different types/amounts of fibers and ground substance.

Three broad categories: 3 main groups:
- Connective tissue proper
- Supporting connective tissue
- Fluid connective tissue
Connective tissue proper subdivides into loose (fewer fibers, more ground substance) and dense (more fibers, less ground substance):
- Loose: Areolar, Adipose, Reticular
- Dense: Regular, Irregular, Elastic
Supporting connective tissue includes:
- Cartilage (Hyaline, Fibrocartilage, Elastic)
- Bone (Compact, Spongy)
Fluid connective tissue: Blood

Resident cells (permanently housed):
- Fibroblasts: produce fibers and ground substance
- Adipocytes: fat cells; clusters form adipose tissue when dominant
- Mesenchymal cells: embryonic stem cells; can differentiate into needed connective tissue cells; act as repair stem cells in adults
- Fixed macrophages: engulf damaged cells/pathogens; release immune signals
- Mast cells: release proinflammatory molecules
Wandering cells: continuously move through tissue; mainly leukocytes and related immune cells; aid in defense and repair (e.g., dendritic cells in dermis)

Collagen fibers: unbranched, cablelike, very strong and flexible; resist stretching; ~25\% of body protein; appear white; pink in H&E; abundant in tendons/ligaments
Reticular fibers: thinner than collagen; same subunits as collagen; form branching networks; provide structural framework in organs like lymph nodes, spleen, liver
Elastic fibers: contain elastin; branch and recoil; yellow fibers; visible with specialized stains; abundant in skin, arteries, and lungs

Ground substance is produced by connective tissue cells; fills ECM alongside fibers; may be viscous, semisolid, or solid depending on tissue
Contains large molecules:
- Glycosaminoglycans (GAGs): ext{glycosaminoglycans}; negatively charged and hydrophilic; attract cations (e.g., Na^+) and water, increasing viscosity
- Types of GAGs: chondroitin sulfate, heparan sulfate, hyaluronic acid
- Proteoglycans: GAGs linked to a protein core
- Adherent glycoproteins: glue that helps attach cells and fibers to ground substance
Ground substance plus fibers form the dynamic extracellular matrix, through which cells and molecules move

Two types in the embryo: Mesenchyme and Mucous (Wharton’s jelly) connective tissue
Mesenchyme: origin of all connective tissues; contains mesenchymal cells and immature protein fibers in a gel-like ground substance
Adult mesenchymal cells act as stem cells to support tissue repair and differentiation into required connective tissue cells